Soon after the development of electrodes having a metal base such as of titanium with an electrocatalytic coating such as of noble metals or their oxides, it was appreciated that coating removal would be desirable for recoating. Very early it was discovered that molten salt baths could be useful for this purpose.
Thus in U.S. Pat. No. 3,573,100 there is disclosed the method for cleaning electrodes using a melt containing an alkaline substance and an oxidizing salt. According to the patent teachings, successful coating removal can be achieved in only a few minutes with these molten salt baths typically heated at 450.degree. C. to 500.degree. C. Similarly, in U.S. Pat. No. 3,684,577 molten salt baths of an alkali metal hydroxide and an alkali metal salt of an oxidizing agent, where the hydroxide is equal to or predominant over the amount of the salt are taught to be useful for electrode coating removal. Again, fast removal times are disclosed.
It was however found that although stripping of the coating by molten salt baths could be accomplished, there could also be achieved a deleterious attack on the base metal. This could readily result in a base metal loss of as much as 5 weight percent. Additionally, recovery of the costly coating constituents from the molten salt bath was reported to be uneconomical.
Other approaches were therefore investigated after these early molten salt bath discoveries. One result was electrode recoating following mere cleaning without stripping of the old coating. Such technique has been disclosed for example in the U.S. Pat. No. 3,684,543. Another result, as discussed in Canadian Patent No. 1,176,600, was the formation of a non-adhesive, intermediate layer between the metal substrate and the electroconductive coating for facilitating subsequent coating removal.
These approaches also included employing solutions for coating removal that could be utilized at more moderate operating temperatures. For example in U.S. Pat. No. 3,761,312 there is taught a coating removal process using an acid or alkaline solution with hydrogen peroxide at a temperature of 60.degree.-80.degree. C. In the companion U.S. Pat. No. 3,761,313 the solution contains certain mineral acid plus hydrofluoric acid or precursor of such acid. Additionally, in U.S. Pat. Reissue No. 28,849 there is taught a method, using an inorganic electrolyte, for electrolytically removing the catalytic coating for the cleaning of the substrate metal.
There is still however a need for coating removal from such coated electrodes that achieves on the one hand preservation of the most desirable surface characteristics of the underlying substrate. On the other hand, the technique used should be able to handle complex electrode configurations, without preferentially attacking portions of the substrate, yet provide for efficient and economical recovery of valuable coating constituents.